Synthetic resins are widely used as engineering plastics in a variety of end-uses, such as building materials and automobile parts. The engineering plastics have good physical and chemical resistance, and are low cost. A disadvantage of some engineering plastics is that they have poor impact strength. Poor impact strength of these materials may be overcome by blending impact modifiers with the resins. Impact modifiers of the core-shell type provide a low-Tg elastomeric core coated with a shell to improve handling and prevent clumping.
Many such additives are known, such as core/shell impact modifiers prepared by emulsion polymerization with a first stage or core of a polymer based on butadiene or on a poly(alkyl acrylate), and with one or more shells or second stages based on polymers which are mainly derived from methyl methacrylate, but which may also have polymer chains derived from vinyl aromatic monomers, such as styrene.
PVC plastics are described having a core/shell impact modifier having a 2-EHA/BA core in U.S. Pat. No. 5,612,413. The patent shows that a copolymer of a low Tg (2-EHA) and high Tg (BA) monomer can provide improved low-temperature performance.
U.S. Pat. No. 5,77,520 describes a core/shell impact modifier for improved low impact performance having an alkyl acrylate core, preferably C5 to C8 alkyl acrylates, blended with a terpolymer impact modifier. N-octyl acrylate is used in the examples.
U.S. Pat. No. 5,360,865 describes a polycarbonate resin having a core-shell impact modifier where the core contains at least 60 percent of a C6 to C10 alkyl acrylate. The preferred alkyl acrylates are n-octyl acrylate, 2-ethylhexyl acrylate and 6-methylheptyl acrylate. The use of a graftlinker and optionally a crosslinker in the core is also described. It is demonstrated that when the number of carbon atoms of an alkyl group in an alkyl ester used in a rubber polymer of the graft-copolymer is increased, the Brittle-Ductile transition of a PC resin containing the modifier can be further lowered.
Impact modification of polymers is a complicated phenomenon. Different resin types have different inherent mechanical properties and require different types of modifiers. For example, it is known that the best modifiers for rigid polystyrene are not the best ones for rigid polymethylmethacrylate or polycarbonate; and the best modifiers for semicrystalline polyolefins are not the best for semi-crystalline polyesters. The optimum toughness can only be achieved through a careful balance of many factors such as modifier particle size, particle size distribution, modifier response under static and dynamic stress environment, and modifier dispersion in the matrix. Polyesters have excellent solvent and chemical resistance and high temperature performance due to their semicrystalline morphology. Polyesters and their alloys with other polymers are widely used in engineering polymer applications.
It has been shown that linear acrylates can achieve better low temperature impact performance than branched acrylate in the core stage of impact modifier preparation. Specifically, a core with n-octyl acrylate can provide excellent low temperature impact toughness. However, an issue with n-octyl acrylate is that it is not widely available.
There is a need in the market for a low-temperature impact modifier with excellent impact toughening properties that is widely available for use in a wide variety of engineering plastics.
It has now been found that an acrylic impact modifier containing predominantly 2-EHA with minor amount of n-OA can provide significantly better low temperature impact performance than when the core is made entirely from 2-ethylhexyl acrylate. This observation is unexpected since both poly(2-ethylhexyl acrylate) and poly(n-octyl acrylate) have identical Tgs. It has also been found that when two monomers with identical Tg are copolymerized, the modifier can provide better toughness than if the modifier is made entirely from one of the modifiers.